The Book of Azrael

In this first installment of the Gods & Monsters series, Dianna made a desperate bargain a thousand years ago to save her dying sister, binding herself to a monstrous figure named Kaden and carrying out his commands—including retrieving an ancient relic from the creatures that hunt her. Meanwhile, a long-vanished king known in the old world as Samkiel and in the new as Liam, called the World Ender, is drawn back into a realm he swore never to return to when those he loves are threatened. As an old enemy resurfaces, the two must set aside their enmity and work together to protect their world and the realms beyond.